1. Field of the Invention
This invention relates in general to a vacuum treatment device and specifically to method of producing a high vacuum in a container which has an interior thin metal sheet spaced from the walls thereof on the inside of the container defining a space therebetween which is connected at one end to an exhaust connection which includes means for introducing a protective gas in the space between the sheet and the interior walls, and for heating the covering for the interior walls.
2. Description of the Prior Art
During the evacuation of a container, the pressure in the high vacuum range depends primarily on the suction capacity of the pumping system and on the amount of vapor (chiefly water vapor) which, during the preceding contact with vapor-containing air, has been sorbed to the inside walls of the container and is slowly released again under the high-vacuum conditions. For this reason, efforts in the high-vacuum technology have continually been directed to maximum possible suction capacity and a minumum possible amount of sorbed vapor.
Opposed to the unlimited increase of the suction capacity, however, are not only the growing costs of larger pumps but also some undesirable consequences related to the vacuum technique. That is, the relative difference between the particle number density in the gas space reached in the container during the pumping, and the particle number density determined by the vapor amount sorbed by the container walls under equilibrium conditions, increases approximately proportionally to the increasing suction capacity. The local differences in the volumetric particle density or in the collision rate by surface within the container, caused by the geometry of the container and any built-in equipment, grow larger in the same proportion. Such differences, however, jeopardize a representative checking of the determinative parameters of the process (for example, the measuring of pressure). Thus, with the augmented suction capacity, the risk increases of unduly affecting the reproducibility of the results of the vacuum process (for example, of the optical properties of thin layers deposited by evaporation in vacuum), even if the differences are very small, such as caused by the spatial arrangement of the built-in equipment, the temperature distribution or temperature variation.
Thus the reduction of the sorbed amount of vapor offers particular advantages over an enhancement of the suction capacity, which advantages, as mentioned above, not only include savings in the size of pumps but are also of a technological nature.
A well-known manner of reducing the sorption is to prevent air from contacting the inside walls of the container and to introduce and remove the pieces to be treated through vacuum-tight air locks. The result obtained, however, does not always justify the high technical expenditures of pressure-tight locks and of auxiliary means for actuating the locks and transporting the pieces to be treated. That is, if during the maintenance or cleaning, the inside walls of the container come even only temporarily in contact with vapor-containing air, the reestablishment of constant process parameters requires a new, relatively long recovery time.
It is known to prevent humid air from penetrating into an open container by flooding the inside walls thereof with a stream of dry gas (protective gas). In this method of the prior art, no pressure-tight locks are needed, however, there is a disadvantage in a large consumption of protective gas.
Also known is to apply a higher, constant temperature, in order to reduce the water adsorption on a container wall. In this method, because of the increased saturation pressure of the water vapor, the relative humidity of the air is reduced (ratio of the partial pressure of the water vapor to the saturation pressure thereof), whereby the amount of sorbed water is also reduced. However, since at a higher temperature the sorbed water amount equilibrates a larger volumetric particle number in the gas space, the obtained effect is relatively small.
Better results are obtained by alternately heating and cooling the inside walls during the evacuation. The temperature is varied periodically in accordance with the cycle of consecutive evacuation processes, predominantly with the aid of fluid heat carriers. During the flooding, the temperature is kept above the dew point given by the air humidity, but mostly not above 60.degree. C., in order not to complicate the maintenance and not to intensify the corrosion.
A further improvement is obtained by heating the container walls during the evacuation to temperatures above 60.degree. C. To accelerate the temperature variation and save energy, it has also been provided to cover the inside walls of the container with heatable protective screens, for example, metal foils, spaced therefrom. The intention was to protect the inside walls from vapor deposition. In this way, particuarly in vacuum coaters, the porous, strongly sorbing vapor-deposited layers can be prevented from forming on the inside walls, while the protective screens coated with such layers may easily be outgassed by heating during the evacuation and thereby regenerated, or they may be exchanged. Experience has been made, however, that the pressure drop thereby obtained in the container is still substantially smaller than that which could theoretically be expected on the basis of the temperature drop.